Human adenoviruses are under intense scrutiny as vehicles for gene therapy and as anticancer agents in humans. Nevertheless, many basic aspects of the mechanisms by which these viruses subvert normal cellular processes to ensure efficient replication remain poorly understood. These include post-transcriptional regulation of viral and cellular gene expression by the adenoviral EIB 55 kDa and E4 Orf 6 proteins, which induce selective export of viral mRNAs from the nucleus to the cytoplasm during the late phase of infection. Those two proteins also exhibit transforming activity, and can induce accelerated degradation of the cellular tumor suppressor protein p53 and block transcriptional activation by p53. The long-term objective of these studies is elucidation of the molecular mechanism(s) by which the adenoviral E1B 55 kDa protein regulates mRNA export, and its relationship to the seemingly disparate functions of the protein in modulation of p53 turnover and activity. The specific aims of this proposal comprise complementary genetic, molecular and biochemical approaches to address these issues. Both mutations targeted to specific sequences and more systematic changes will be introduced into the E1B 55kDa protein coding sequence in the viral genome, and a wide range of phenotypes exhibited by the mutant viruses examined in both transformed and normal human cells. These studies will provide a detailed description of the protein, as well as mechanistic information. Identification of sequences required for mRNA export regulation will facilitate molecular and biochemical investigations of the mechanism(s) by which the EIB protein induces selective export of viral mRNAs. These studies will employ a variety of methods to address such questions as which cellular RNA export pathway(s) are subverted by the viral protein, whether its binding to RNA in vivo is necessary for selective mRNA export and whether the protein participates in, or is required for, the reorganization of nuclear components that has been correlated with efficient export of viral mRNAs.